The present invention relates to a method of navigating along a hollow tubular object such as a biological object with a lumen, the object included in a three-dimensional volume data set representing a biological structure.
Patient imaging methods, such as computer-assisted tomography (CT), magnetic resonance imaging (MRI), ultrasound and positron-emission tomography (PET), generate large three-dimensional volume data sets representing all or part of a patient's body. These volume data sets are highly detailed and allow complex studies of the body to be made. Techniques for this include virtual endoscopy, in which a virtual camera is made to travel along a biological object with a lumen such as a colon, blood vessel or bronchial tree, with the image of the lumen that is seen by the camera presented to a user for diagnostic and other purposes. To navigate the lumen, a path for the camera to follow is often determined wholly or partially in advance, since even an experienced clinician may have difficulty in manually controlling the camera movement.
A number of methods have been proposed for controlling the camera and calculating suitable camera paths. A real-time user experience that is at least partly automated is preferred for virtual endoscopy, and important criteria for achieving this include speed, simplicity of computation, and wall avoidance for the camera. It is not generally trivial to satisfy all the criteria, and previously proposed methods are many and varied. Some provide fully automated navigation, in which the entire camera path is calculated in advance, followed by processing to generate the images shown by the camera during its journey. Other methods calculate a navigation direction and associated camera images a little at a time. This approach offers some user interaction as the camera progresses along the lumen but it is more difficult to give a high-quality real-time viewing experience owing to the time needed to produce the images.
Published methods for automated or semi-automated navigation include those based on mathematical techniques designed for wall avoidance in generating a path from a start point to an end point [1, 2]; those based on mathematical techniques that use erosion to determine a centerline along the lumen [3]; labeling points in the data set with their distance from the end of the path using a wavefront model to avoid obstacles and calculating the path by moving from point to point according to the closest distance to the end point [4]; obtaining an initial path by using a distance label map to connect start and end voxels in the data set via intermediate voxels according to their distance from the start and end voxels, and then centering the path using maximum diameter spheres [5, 9]; and determining navigation steps by using ray casting to find the longest ray from a camera position to the wall and weighting this with the existing viewing direction of the camera to obtain a new direction for movement of the camera, and repeating this for each step [6].
General data collection and processing methods for viewing three-dimensional patient images have also been described [7, 8], including virtual endoscopy in which the user navigates using a computer mouse or in which the user's viewpoint is automatically moved through the data set along the centerline of a lumen. The centerline is determined using erosion or by calculating the centers of a series of planes of minimum area dissecting the lumen.
A number of commercial virtual endoscopy systems and the various approaches taken to camera navigation and to the image processing necessary to represent a realistic view of the lumen are discussed by Bartz [10].
The present invention seeks to provide an alternative method for navigating a lumen, such as for navigation of a virtual endoscopy camera.